Electrical Impedance Tomography (EIT) is an imaging methodology that is based upon electrical conductivity or impedance contrasts within an object. U.S. Pat. No. 4,539,640, issued Sep. 3, 1995, to inventors Bradley Fry and Alvin Wexler, and the article by A. Wexler, B. Fry and M. R. Neuman, entitled “Impedance-Computed Tomography: Algorithm And System”, Applied Optics, Vol. 24, No. 23, pp. 3985-3992, describe a method and embodiment of a system that solved electromagnetic field equations that govern current flow in a conductive medium, and concurrently extracted an image of the interior of the medium based on the electric current conductivity (and, more generally, specific impedance) distribution in the medium. This provided a methodology for the correct mathematical solution of the inverse (imaging) problem and construction of electronic equipment for this purpose.
Whilst EIT has frequently been applied to observe bulk changes in electrically heterogeneous media to detect relatively large changes in static objects, it does not allow for rapid detection of relatively small changes in the electrical impedance of a fluid.
Furthermore, existing acoustic sensing arrays can only provide a 2D slice or surface through the sound field, and as such cannot extend into the depth of the fluid to provide a 3D measurement. Whilst it would be possible to deploy a full 3D array of microphones, this array is likely to influence the sound field, and would require a large measurement array that protrudes into the depth of the fluid being imaged. This technique allows for detection at a distance away from the target.